polymers Review Thermoplastic Pultrusion: A Review Kirill Minchenkov, Alexander Vedernikov, Alexander Safonov * and Iskander Akhatov Skolkovo Institute of Science and Technology, Center for Design, Manufacturing and Materials, 30/1 Bolshoi Boulevard, 121205 Moscow, Russia; [email protected] (K.M.); [email protected] (A.V.); [email protected] (I.A.) * Correspondence: [email protected] Abstract: Pultrusion is one of the most efficient methods of producing polymer composite structures with a constant cross-section. Pultruded profiles are widely used in bridge construction, transporta- tion industry, energy sector, and civil and architectural engineering. However, in spite of the many advantages thermoplastic composites have over the thermoset ones, the thermoplastic pultrusion market demonstrates significantly lower production volumes as compared to those of the thermoset one. Examining the thermoplastic pultrusion processes, raw materials, mechanical properties of thermoplastic composites, process simulation techniques, patents, and applications of thermoplastic pultrusion, this overview aims to analyze the existing gap between thermoset and thermoplastic pultrusions in order to promote the development of the latter one. Therefore, observing thermoplastic pultrusion from a new perspective, we intend to identify current shortcomings and issues, and to propose future research and application directions. Keywords: thermoplastic pultrusion; thermoplastic composites; fiber-reinforced materials 1. Introduction Today, polymer composite materials have found wide application in various indus- tries [1,2]. This was made possible by extensive studies conducted over the last 70 years [3,4]. Citation: Minchenkov, K.; The popularity of composite materials results from their properties, such as high specific Vedernikov, A.; Safonov, A.; strength and stiffness [5–8]; improved durability [9,10]; high fatigue [11,12], chemical [13,14], Akhatov, I. Thermoplastic Pultrusion: and corrosion resistance [15–17]; and ease of transportation and assembly [18–20] of composite A Review. Polymers 2021, 13, 180. https://doi.org/10.3390/polym structures. Composite materials are produced by various processes, e.g., autoclave molding, 13020180 resin transfer molding, compression molding, filament winding, and pultrusion [21,22]. Pultrusion is a process where a pack of reinforcement fibers impregnated by resin is pulled Received: 12 November 2020 through a heated die block, where the polymerization process takes place [23]. This method Accepted: 30 December 2020 allows fabrication of products having constant cross-section [24,25]. The advantages of Published: 6 January 2021 pultrusion over other composite manufacturing processes are its high production rate of up to 5 m/min [1], higher efficiency [26,27] and low costs [28,29] of production, and the Publisher’s Note: MDPI stays neu- ability to produce profiles of virtually indefinite length [30]. There are thermoplastic and tral with regard to jurisdictional clai- thermoset matrix-based composites [31,32]. Thermosets are nonmelting polymers obtained ms in published maps and institutio- during chemical reaction (polymerization) between a resin and a hardener, while thermo- nal affiliations. plastic composites can change their state and melt under heating. Fiber reinforcement is impregnated by hot melt thermoplastic polymer; then, after cooling, the part is ready for use. Compared to thermosetting composites, thermoplastic composites have higher impact toughness [33–36], are faster to produce [37,38], have higher service temperatures [39], Copyright: © 2021 by the authors. Li- censee MDPI, Basel, Switzerland. can be joined by welding [40–43], have less environmental impact [44–46], and can be recy- This article is an open access article cled [47–50]; their source materials have virtually unlimited shelf life [51–57]. Pultruded distributed under the terms and con- thermoplastic profiles are used in various structures and sectors, such as vehicles [58–63] ditions of the Creative Commons At- and aircrafts construction [64–66], aerospace [67–69] and civil engineering [70–73], en- tribution (CC BY) license (https:// ergy systems [74], restoration of deteriorated structures [75], marine applications [76–79], creativecommons.org/licenses/by/ oil and gas industries [80], electromagnetic interference shielding elements [81,82], window 4.0/). profiles [83], pipes [84,85], rebars [86,87] and rods [88–91]. Polymers 2021, 13, 180. https://doi.org/10.3390/polym13020180 https://www.mdpi.com/journal/polymers Polymers 2020, 12, x FOR PEER REVIEW 2 of 36 Polymers 2021, 13, 180 rated structures [75], marine applications [76–79], oil and gas industries [80], electromag-2 of 36 netic interference shielding elements [81,82], window profiles [83], pipes [84,85], rebars [86,87] and rods [88–91]. Today, the number of studies and publications in the field of thermoset pultrusion is Today, the number of studies and publications in the field of thermoset pultrusion is an an order of magnitude larger than the number of those in thermoplastic pultrusion, alt- order of magnitude larger than the number of those in thermoplastic pultrusion, although hough advantages offered by thermoplastic composites provide enough reason for deeper advantages offered by thermoplastic composites provide enough reason for deeper study. study. From industrial point of view, it is worth noticing that, for instance, Fiberline Com- From industrial point of view, it is worth noticing that, for instance, Fiberline Composites posites A/S, being one of the largest companies in the pultrusion manufacturing and the A/S, being one of the largest companies in the pultrusion manufacturing and the world’s world’sbiggest web-shopbiggest web-shop of fiberglass of fiberglass profiles [ 92profiles], has numerous[92], has numerous types of pultruded types of pultruded structural profilesstructural available profiles foravailable purchase for withpurchase all of wi themth all being of them thermoset being ones.thermoset Several ones. questions Several questionsarise in this arise connection. in this connection. Why are there Why no are thermoplastic there no thermoplastic profiles available profiles for the available customers for theto purchase, customers despite to purchase, their numerous despite their advantages? numerous Why advantages? is there a Why well-developed is there a well-de- market velopedfor thermosetting market for profiles thermosetting and almost profiles no market and almost for thermoplastic no market fo ones?r thermoplastic ones? Currently, there is nono reviewreview on thermoplasticthermoplastic pultrusion. By analyzing the thermo- plastic pultrusion process from different perspectives (techno (technology,logy, raw materials, proper- proper- ties, numerical modeling, applications, etc.) an andd recalling the main publications regarding thermoplastic pultrusion (scientific (scientific articles, patents),patents), this article aims to understand why thermoplastic pultrusion failed to receive the attention and broad acceptance it deservesdeserves from thethe scientificscientific andand engineering engineering community, community as, as opposed opposed to thatto that of thermosetof thermoset pultrusion. pultru- sion.Exploring Exploring this issue, this issue, the authors the authors intend intend to attract to attract scientists’ scientists’ attention attention and stimulate and stimulate further furtherdevelopments developments in thermoplastic in thermoplastic pultrusion. pultrusi Sectionon. Chapter2 describes 2 describes the process the process of thermo- of ther- plasticmoplastic pultrusion pultrusion and and components components of pultrusionof pultrusion machines, machines, provides provides the the classification classification of ofthermoplastic thermoplastic pultrusion, pultrusion, analyzes analyzes basic basic parameters parameters of of pultrusion pultrusion process, process, andand reviewsreviews patents registeredregistered andand future future research research possibilities. possibilities. Section Chapter3 discusses 3 discusses raw materialsraw materials used usedin manufacturing in manufacturing of thermoplastic of thermoplastic composites, composites, mechanical mechanical properties properties of the of pultruded the pul- profiles,truded profiles, and promising and promising areas for areas the further for the investigations.further investigations. Section4 Chapter reviews 4 the reviews methods the methodsof process of modeling process modeling and discusses and discusses of possible of directions possible directions for scientific for work.scientific Section work.5 Chaptersanalyzes the5 analyzes possible the applications possible applications of thermoplastic of thermoplastic profiles, patents profiles, registered, patents andregistered, future andresearch future and research application and application possibilities. possibilities. 2. Thermoplastic Pultrusion and Process Parameters Luisier et al. were the firstfirst to proposepropose thethe classificationclassification ofof thermoplasticthermoplastic pultrusionpultrusion processes in two groups [[93].93]. The firstfirst is nonreactivenonreactive thermoplastic pultrusion where the process is based on the already polymerized materials, as opposed to the second one— reactive thermoplastic pultrusion where thermoplasticthermoplastic is polymerizedpolymerized duringduring chemicalchemical reaction betweenbetween thermoplasticthermoplastic resin resin and and catalyst/activator,